The onset of nighttime melatonin production, as measured by gas chromatography - negative chemical ionization mass spectrometry, is a potentially important "biological marker" for 24-hour (circadian) and seasonal rhythms and the hypothalamic effects of light on these rhythms. The purpose of this project is to assess the timing of circadian rhythms and the effects of light in normal human subjects, using the timing (phase) of the nighttime increase in melatonin production as a maker for circadian phase position. The timing of the first REM period and the core body temperature circadian rhythm will also be examined. After determination of baseline circadian phase position, dynamic (phasic) responses to light will be studied by measuring the change in phase position of these markers after shifting the times of "dawn" and "dusk" using bright artificial light. Light sensitivity will also be assessed by measuring the decrease in melatonin levels during exposure to bright light in the middle of the night. Subjects will also be studied throughout the year to examine seasonal rhythms in light sensitivity, circadian phase position, and the pattern and amplitude of melatonin production. In addition to normal subjects, patients (suspected of having abnormal biological rhythms) with endogenous depression and "winter depression" will be studied. These studies are among the first to evaluate the effects of bright light on biological rhythms in humans and may lead to improved diagnosis and treatment of certain types of sleep and mood disorders.